1. Field of the Invention
The present invention relates to water resistant ink compositions which are highly suitable for use in recording methods that employ water-based inks, especially ink-jet printing. These compositions are water resistant in that after having been printed onto a recording medium and dried, no loss in quality occurs whatsoever of the printed image even upon accidental exposure to water or beverages, and also have excellent color stability.
2. Prior Art
Ink-jet printing makes use of a number of different ink discharge techniques to form droplets of ink and deposit some or all of the droplets on a recording medium such as paper ("ink" being understood here to refer to a recording liquid). Outstanding techniques include electrostatic attraction, the application of mechanical vibrations or changes to the ink using a piezoelectric element, and thermal bubble formation from the ink combined with use of the resulting pressure. These ink-jet printing methods are in wide use today because they generate little noise and enable high-speed, multicolor printing to be carried out.
The inks commonly used in ink-jet printing contain water as the main component to ensure safety and good recording characteristics. Polyhydric alcohols are also included to prevent clogging of the nozzles in the ink-jet printing system and to enhance discharge stability.
However, further improvement needs to be made in the properties of ink compositions in order to keep pace with the better performance and more widespread use recently of ink-based recording systems and related technology. This need for improvement is particularly acute with regard to the water resistance of inks. The past few years have seen a very rapid rise in the use of ink-jet printers, both in the office and at home. However, in the home and office environment, there is a greater chance of printed matter coming into contact with water or beverages such as coffee or soda. Under such conditions, it is essential that the ink be water resistant.
Yet, very few if any of the water-soluble inks with good color development currently on the market are water resistant. When a printed image comes into contact with water, for instance, the image is almost always ruined by color bleeding or the like. Because both the dye itself and the polyhydric alcohol added to improve the discharge stability of the ink are water-soluble, direct contact of the printed image with water allows these components to dissolve out into the water, destroying the quality of the image.
Of course, the polyhydric alcohol component of a dye-containing ink may be eliminated, but the result is a loss in discharge stability.
An attempt to confer reactivity on the dye itself so that it is bound to the substrate has been reported in Shikizai, 67, No. 6, 356-361 (1994). The dye is made reactive by introducing alkoxysilyl groups onto the dye molecule. Unfortunately, water-soluble dyes often have polar groups such as --SO.sub.3 Na, --NH.sub.2, and --CN incorporated therein to confer water solubility. These polar groups tend to react with the alkoxysilyl groups, making it difficult to successfully introduce alkoxysilyl groups. Moreover, the resulting dye has a poor stability in aqueous solutions, and is also unduly expensive.
According to another method, described in Shikizai, 66, No. 9, 517-522 (1993), a dye is added to tetraethoxysilane or methyltriethoxysilane to form a sol, which is applied to a glass substrate, whereby the dye is immobilized. However, because this type of sol normally gels immediately upon addition to a basic water-based ink, it cannot be used in water-based inks.
Silicone compounds are commonly used as additives for imparting water resistance. While these do indeed confer excellent water resistance in a solvent system, their stability in water is rather poor. At best, they are somewhat stable under weakly acidic conditions, but generally lose all stability when the system is even mildly alkaline. Given that water-soluble dyes are most often alkaline, silicone compounds are of no use in such a system.
Water-soluble polyamines have been used in quite a few cases recently to provide dyes with water resistance. However, one drawback has been intense color changes which alter the color tone of the dye, possibly due to the decomposition of azo groups in the dye on account of the reactivity and basicity of the amino groups, and especially the primary amino groups, on the polyamine.
JP-A 279678/1994 describes a water-based surface treatment composition comprising as a primary component either the hydrolyzate of the product obtained by reacting an aminotrialkoxysilane with an organic compound that reacts with the amino group on the silane, or the product obtained by further reacting this hydrolyzate with an organometallic compound such as a tetraalkoxysilane. Amino groups are blocked to some extent by using polydiglycidyl compounds as the blocking agent. Although the blocking of amino groups limits changes in the color tone to some degree, the use of polydiglycidyl compounds destabilizes the liquid itself, such as by making it subject to gelation. This method also calls for the addition of an anionic surfactant as the stabilizer, which can be detrimental to the water resistance. Another problem has been the instability of this liquid under alkaline conditions, making it of little use as an additive in water-soluble ink compositions.